The present invention relates to a sound producing system particularly adapted to create a dimensionalized impression of the sound.
In a typical stereophonic sound reproduction system, where there are two speakers, a "dimensionalized" effect can be obtained by transmitting different sound signals to the two speakers. Thus, with the speakers being positioned at two laterally spaced locations, and the listener being positioned rearwardly of the speakers and facing toward a location between a speakers, a distinct sound that is transmitted only from the left speaker can be detected by the listener as coming from that source since the left and right ears of the listener will detect a difference in intensity and also detect a phase shift so as to obtain the impression of the direction of the sound. When another distinct sound is transmitted from the right speaker the direction of that source of sound can also be detected by the listener. Thus, the sound can be expanded to the area encompassed by the two speakers.
There have been attempts in the prior art to give even greater dimension to the sound reproduction system, so that there is the impression that the sound is coming from areas totally outside of the more limited area at and between the two speaker locations. While the applicant is not totally familiar with the operation of these systems, according to the applicant's present understanding, such systems require rather limited conditions of operation. For example, it is known that the prior art systems known to the applicant must be utilized in an environment where there is very little reflected sound, for example in an open space, or in a room where the walls are made of a highly sound absorbent material. Further, the systems which are known to the applicant are quite sensitive to the location of the hearer's head. Thus, if the person moves his head from a precise listening location, or rotates his head moderately toward one speaker or the other, a large part of the dimensionalized effect is lost. Thus, to the best knowledge of the applicant, these systems have remained more in the category of laboratory curiosities, rather than a system which is practical for general use.
It is believed that the prior art systems discussed immediately above are operated on the basis of recognizing that sound emanating from various locations both forwardly and rearwardly of a person's head create different sound patterns relative to the person's ears. Thus, a sound emanating from a location in front of the person and 30.degree. to the left would produce distinctly different relative sound patterns to the person's ears than a sound emanating directly from a location at the person's left. There would be a difference in intensity for the various frequencies, and also a different phase shift detected by the person's two ears. It is believed that this phenomenon is utilized to tailor or control the sound emanating from the speakers to cause delicate adjustments in the phase shift and sound intensity at different frequencies to produce the effect of greater dimensionalized sound. However, as indicated above, it is believed that the sensitivity of such systems to reflected sound and also head location have not made them practical for general use.
Also, there has been in the prior art recognition of the phenomenon called "cross talk" which in certain circumstances has the effect of degrading the quality of the sound transmitted from two spaced speakers. To describe this phenomenon briefly, the sound from a right speaker reaches both the right and left ear of the person, but reaches the left ear at a slightly later time depending on the distance between the speakers, the listening angle, and the ear spacing of the listener (e.g. at a time ranging from zero to 900 microseconds) and at a somewhat lower intensity than the sound which reaches the right ear. The sound from the left speaker acts in somewhat the same way relative to the left and right ears. With similar sounds being emitted from both speakers something of the stereophonic effect is lost or at least diminished by this phenomenon of cross talk.
This problem was recognized in U.S. Pat. No. 4,058,675, and this patent disclosed a system which has for its intended purpose the elimination of the deteriorating effect of cross talk. Since it is believed that a deeper understanding of the apparatus of U.S. Pat. No. 4,058,675 will aid in a fuller appreciation of the present invention, the apparatus will be discussed in some detail herein.
Reference is made to FIG. 1, which shows a left speaker LS and a right speaker RS at two spaced locations, and a person P at a listening location equally distant from the speakers LS and LR and located rearwardly of the speakers. The person has a left ear Le and a right ear Re.
The sound from the left speaker can be considered as having two components, namely component Ll which is transmitted from the left speaker LS to the left ear Le, and a second component Lr which is transmitted from the left speaker LS to the right ear Re. The right speaker in like manner has two sound components Rr and Rl transmitted to the right ear Re and left ear Le, respectively.
In U.S. Pat. No. 4,058,675, there is a discussion of the effect of cross talk in that the sound reaching the right ear Re from the right sound component Rr reaches the right ear shortly before the left sound component Lr reaches the right ear Re. Thus, if substantially the same sound is being transmitted from the left and right speakers, the right ear will hear the sound first at a higher intensity, and the same sound with a slightly delayed phase shift at a lower intensity. U.S. Pat. No. 4,058,675 proposes to alleviate this problem by providing additional left and right auxilliary speakers to provide cancelling sounds to eliminate cross talk. This will be explained with reference to FIG. 2, labelled "Prior Art" and corresponding to FIG. 5 of U.S. Pat. No. 4,058,675.
It can be seen that there is a left speaker LSP, made up of a main left speaker MSL and a left subspeaker SSL. The left signal L enters at the terminal "IN", and is transmitted directly to the main left speaker MSL. In addition, the left signal is applied through an attenuator AT and phase shift and delay means DP to the subspeaker SSL. In like manner, the right speaker SPR has a main speaker MSR and subspeaker SSR, along with an attenuator AT and delay means DP.
In the operation of the device of U.S. Pat. No. 4,058,675, the main sound component Ll from the main left speaker MSL reaches the left ear Le substantially undiminished. Also, there is a main sound component (not shown herein for clarity of illustration) from the main right speaker MSR to the right ear Re. However, the second sound component Rl from the right main speaker reaches the left ears El slightly later than the main right sound component reaches the left ear and also later than the left main sound component Ll reaches the left ear. The delayed and inverted signal SLl from the left subspeaker SSL is timed at a predetermined phase signal and directed at a predetermined intensity to substantially cancel the right second component Rl. Thus, the cross talk from the right speaker MSR is substantially attenuated. The operation is substantially the same with respect to the right ear, so that the left ear hears sounds mainly from only the left speaker, while the right ear hears sounds mainly from the right speaker.
While the applicant has not conducted an exhaustive analysis of the device shown and described in U.S. Pat. No. 4,058,675, the analysis and limited experimental evaluation which was done indicates that such apparatus has significant limitations in producing a truly dimensionalized sound effect. The reasons for this will become more apparent from the following detailed description.
In view of the foregoing, it is an object of the present invention to provide a method and apparatus to produce a greater dimensionalized sound effect, which method and apparatus would be practical for general use.